Dopamine D2 receptor blockade is currently the main mechanism through which most antipsychotic medications (AP) are thought to work. However, both dopamine and glutamate in the prefrontal cortex appear to have an important role in the symptomatology of schizophrenia (Weinberger, Berman et al. 1986; Carlsson and Carlsson 1990; Wang and O'Donnell 2001).
Atypical antipsychotics appear to be more effective than classical antipsychotics in treating schizophrenia negative symptoms and cognitive dysfunction (Bhana, Foster et al. 2001; Javitt 2001). Interestingly, compared to classical AP, atypical AP preferentially increase glutamate (Daly and Moghaddam 1993; Yamamoto, Pehek et al. 1994) and dopamine release (Nomikos, Iurlo et al. 1994; Westerink, de Boer et al. 1998; Watanabe and Hagino 1999) in the medial prefrontal cortex.
By itself, the dopamine hypothesis cannot account for the observation that several weeks of antipsychotic treatment are required for what may be suboptimal clinical response, and why negative symptoms often do not improve with typical antipsychotics. Paradoxically, stimulant treatment can actually improve the negative symptoms of schizophrenia (Angrist, Peselow et al. 1982; van Kammen and Boronow 1988). The dopamine hypothesis has evolved into a theory of cortical-subcortical dopamine imbalance, with excess subcortical dopamine function associated with positive symptoms, and dopaminergic hypoactivity underlying negative symptoms (Weinberger, Berman et al. 1988; Davis, Kahn et al. 1991). Cortical and subcortical structures are functionally linked via the neurotransmitter glutamate, so disruption of glutamatergic neurotransmission could lead to the dissociation of dopaminergic function in these regions predicted by this model (Carlsson and Carlsson 1990). The involvement of glutamate in the pathophysiology of schizophrenia is further supported because there is pharmacological evidence that glutamatergic hypoactivity may itself be associated with the illness (Meador-Woodruff and Healy 2000).
The schizophrenia-like syndrome generated by antagonists such as phencyclidine (PCP) to the NMDA glutamate receptor resembles the disorder even more closely than that induced by dopamine agonists, as it includes negative symptoms as well as cognitive deficits. Pharmacological agents that act on glutamatergic receptors differentiate themselves from more traditional antidopaminergic agents in their ability to improve negative symptoms. Antipsychotic drugs in general can affect glutamatergic neurotransmission at clinically therapeutic concentrations by altering the presynaptic release of excitatory amino acid and also the density or subunit composition of glutamate receptors (Arvanov, Liang et al. 1997; Arvanov and Wang 1998; Chen and Yang 2002) The glutamatergic effects of antipsychotics are importantly concentration-dependent and, depending on their relative dose-response curves, different agents may act either as agonists or antagonists at therapeutic concentrations.
Growing evidence suggests that the effects of certain atypical antipsychotics on NMDA receptors may be a key differentiation of these agents from conventional antipsychotics. Corbett et al. (Corbett, Camacho et al. 1995) reported that olanzapine and clozapine, but not haloperidol or risperidone, reversed PCP-induced social withdrawal in rats. Olanzapine, clozapine, and fluperlapine strongly prevented the neurotoxicity induced by NMDA receptor antagonists, whereas haloperidol and thioridazine displayed intermediate effectiveness (Farber, Price et al. 1993; Olney 1994; Olney and Farber 1994; Farber, Foster et al. 1996). Both the selective 5-HT2A antagonist M100907 and clozapine prevented PCP-induced blockade of NMDA receptors (Wang and Liang 1998), whereas selective D2 blockers had no effect. Lidsky et al. (Lidsky, Yablonsky-Alter et al. 1993) found that clozapine, and not haloperidol displaced the ligand [3H]MK801 from the NMDA receptor at therapeutic levels. Arvanov et al. (Arvanov, Liang et al. 1997) found that clozapine but not haloperidol produced an enhancement of NMDA-receptor-mediated neurotransmission.
Consistent with these results, Ninan et al have demonstrated that atypical antipsychotic drugs markedly facilitate NMDA- and electrically evoked, but not AMPA-evoked responses in pyramidal cells of the rat medial prefrontal cortex (Arvanov and Wang 1998; Jardemark, Ai et al. 2001; Ninan and Wang 2003). This might account for the ability of atypical antipsychotics to reduce schizophrenic negative symptoms and in improving neuropsychological and cognitive functions (Hagger, Buckley et al. 1993; Beasley, Tollefson et al. 1997; Tollefson and Sanger 1997).
Since glycine and serine can bind to the modulatory site on NMDA receptors, they also can be used to investigate the relationship between antipsychotic action and NMDA receptor function. Chronic administration of a variety of antipsychotic drugs, both typical and atypical, affects glycine-stimulated NMDA receptor binding in several brain areas (McCoy and Richfield 1996). In the absence of glycine, haloperidol augmented NMDA receptor activity, but raising the extracellular glycine concentration reduced the effect of haloperidol, which therefore appears to be a partial agonist at the glycine binding site associated with the NMDAR complex (Lidsky, Yablonsky-Alter et al. 1997). Several studies reported results that were consistent with this interpretation (Waziri 1988; Rosse, Theut et al. 1989; Costa, Khaled et al. 1990; Fletcher and MacDonald 1993) (Javitt, Zylberman et al. 1994; Heresco-Levy, Javitt et al. 1996; Meador-Woodruff, King et al. 1996).
In a related approach, several groups have administered D-cycloserine, an antitubercular drug that acts as a relatively selective partial agonist at the glycine modulatory site (Watson, Bolanowski et al. 1990). D-cycloserine significantly improved negative symptoms when added to conventional antipsychotics, but it did not improve performance on a cognitive battery. D-cycloserine added to clozapine resulted in worsening of negative symptoms (Goff, Tsai et al. 1996; Goff, Tsai et al. 1999; Goff and Coyle 2001).
Several investigators have found that chronic antipsychotic treatment alters the expression of mRNA encoding glutamate receptor subunits, which varies depending on the drug type, the subunit, and the brain region (Meshul and Tan 1994; Meador-Woodruff, King et al. 1996; Healy and Meador-Woodruff 1997; Riva, Tascedda et al. 1997). In general, conventional antipsychotics increased the amount of mRNA encoding NMDA receptor subunits (NR1 and NR2 proteins) in the striatum, whereas clozapine treatment produced no change (Riva, Tascedda et al. 1997). This difference may be involved in the differential liability for extrapyramidal side effects during typical versus atypical AP treatment.
An important problem facing clinicians in the treatment of schizophrenia is the significant variability in clinical response to antipsychotic medications. At least one third of patients treated with traditional APs do not respond well, and only 30-60% of individuals resistant to typical antipsychotics may demonstrate a beneficial clinical response to clozapine with respect to positive and negative symptomatology (Basile, Masellis et al. 2002).
There is strong evidence to suggest that genetic variation plays an important role in inter-individual differences in medication response and toxicity. Molecular genetic approaches provide a novel method of dissecting the heterogeneity of psychotropic drug response. These pharmacogenetic strategies offer the prospect of identifying biological predictors of psychotropic drug response and could provide the means of determining the molecular substrates of drug efficacy and drug-induced adverse events. (Malhotra, Murphy & Kennedy, 2004).
Several studies have been conducted investigating the role of various receptor gene polymorphisms in association with response to clozapine. A few experiments have been conducted on the NR2B subunit gene (GRIN2B) polymorphisms in association with clozapine response: Hong et al (Hong, Yu et al. 2001) found positive association between the C2664T polymorphism and higher clozapine dosage in 100 Chinese treatment refractory patients. These results were replicated by Chiu et al (Chiu, Wang et al. 2003).
There is a need in the art to identify tests or assays that are predictive of a subject's response to drug therapy. Further, there is a need in the art for methods of identifying DNA polymorphisms that may be used to predict a patient's response to drug therapy. In addition, there is a need in the art to identify specific polymorphisms in DNA sequences that are predictive of negative symptom change during clozapine treatment.